﻿Cooling exhibited by Glass and by Steel. 187 



ferred to diminution of thickness. Unfortunately it is not 

 easily possible to discriminate between the effect of diameter 

 and the possibly concomitant effect of lessened birefractive 

 power. Hence these experiments fail to indicate whether the 

 removal of consecutive shells by solution is accompanied by 

 changes of strain (shrinkage). If the fragments of a shattered 

 P. E. drop submerged in glycerin be examined under the 

 microscope between crossed nicols, the presence of strain in 

 many of the individual splinters is clearly apparent. It is well 

 to note that marked polariscopic evidence of strain remains long 

 after the explosive properties of the drop have disappeared. 

 We infer that the electrical behavior (resistance) of the succes- 

 sive cores of a hard steel cylinder is not unexemplified by the 

 optical behavior of successive cores of a P. E. drop ; though 

 it is difficult to say whether the behavior of the partial bodies 

 in either case (partial strains) is at all comparable with the be- 

 havior of the bodies as a whole (temper strain). 



In this place it is well to advert to certain important data of 

 Table VI. Having removed two shells (total thickness #!+#, 

 =0*04) from each of the P. E. drops, Nos. 10 and 11, the 

 density of the cores were found to be 2*4166 and 2*4147, re- 

 spectively. We now annealed them in sulphur (450°) and 

 found for the densities of the annealed cores 2*4263 and 2*4261, 

 respectively. This increment of density is quite as large as is 

 observed when the drops are annealed as a whole.* Hence we 

 fail to appreciate any diminution of strain due to solution. 

 Moreover we prove below that the increment of density ob- 

 served on annealing at the said temperature 450° is a true 

 increment of the density of the substance of the drop ; that it 

 is not due to a partial collapse of the bubbled P. E. drop in 

 virtue of atmospheric pressure. The observed u after -action" 

 in other words is inherent in the glass itself, as we pointed 

 out elsewhere.*)* 



Distribution of density in P. R. drops. 



Results. — Availing ourselves of the property of the P. E. 

 drop to dissolve quietly in hydrofluoric acid, we obtained the 

 following results (Tables III and IY) for the variation of dens- 

 ity along transverse radii of the drops. In these tables " Diam- 

 eter " denotes the mean transverse thickness of the successive 

 cores ; M^ A w their mass and density (0° C), respectively. 

 Furthermore [i, #, <5, R denote the mass, thickness, density and 

 mean radius, respectively, of the consecutive shells. If the 

 nth core be left after the removal of n shells, then the suffixes 

 to the number of the P. E. drop in column first give the his- 



* L. c, p. 446. f This Journal, III, xxxi, p. 452. 



